Atypical odontalgia (AO), also known as phantom tooth pain and trigeminal deafferentation pain, is defined by the International Association for the Study of Pain as a severe throbbing pain in a tooth without major pathology. A diagnosis of AO is made when persistent oral pain occurs in the absence of dental pathologies or when pain persists after a dental treatment would be expected to eliminate the source. A history of previous regional invasive procedures is found in one third of AO Patients. The most common dental procedure associated with AO is root canal treatment (RCT) epidemiological studies demonstrated that 3-12 % of patients undergoing successful endodontic treatment develop AO. The resemblance to toothache and the lack of an accurate diagnostic test for AO often leads to unnecessary dental treatments and delayed diagnosis. Increasing evidence supports Quantitative Sensory Testing (QST) as an effective method for assessment of nerve damage and the inflammatory and central processing mechanisms associated with neuropathic pain. The purpose of this study is to evaluate the efficacy of QST for diagnostic assessment of AO and to begin to evaluate some of the underlying etiological mechanisms and possible heterogeneity underlying this disorder. Three types of patients will be evaluated using QST, a pain and risk factor questionnaire and genetic polymorphism assays: patients suffering from persistent oral deafferentation pain following previous root canal treatment and already diagnosed as AO (n = 50);patients without a previous history of AO undergoing root canal treatment evaluated prior to and one year after the treatment (n = 100);healthy controls (n = 100;recruited from volunteered staff and students, building a similar age and gender group). QST assessments will differentiate between large, thickly myelinated (A2) fibers, thinly myelinated (A4) and unmyelinated (C) fibers. In addition we will employ dynamic sensory tests as temporal summation and Diffuse noxious inhibitory control (DNIC) for central nervous system involvment evaluation. DNA will be extracted from saliva and used to assay polymorphisms in a limited number of candidate genes that have been shown in previous human or animal studies to be associated with differences in susceptibility to acute and/or neuropathic pain. AO patients (50 existing cases plus ca. 5, new cases expected to occur in our prospective study of 100 root canal patients) will be compared to the ca.95 root canal patients who do not develop AO and the 100 matched healthy controls. The proposed study findings can outline the genotype and sensory phenotype of AO patients and may suggest traits of patients undergoing RCT that are prone to develop AO. This study results may serve as the ground for a further study that will evaluate more patients prior to the procedure, will study predisposing factors and will help predict treatment outcome. Project Narrative: Atypical odontalgia (AO), also known as phantom tooth pain and trigeminal deafferentation pain, is defined by The International Association for the Study of Pain as a severe throbbing pain in a tooth without major pathology. The resemblance to toothache and the lack of an accurate diagnostic test for AO often leads to unnecessary dental treatments and delayed diagnosis. The goal of the proposed project is to compare sensory characteristics of AO, symptomatic and non-symptomatic root canal treatments (RCT). We will follow the RCT group for up to a year following the treatment. The characteristics of the persistent pain will be evaluated and compared with the diagnosis, amplitude and patterns of AO. Quantitative Sensory Testing (QST) has been shown to differentiate between, perineural inflammation, nerve damage and centrally mediated pain. Employing those tests for patients suffering from AO and following root canal treatment (The most common dental treatment related to AO) may demonstrate sensory changes that could improve our AO mechanisms understanding. Ideally, QST may become a valuable diagnostic tool for this condition, predict AO development following root canal treatment and to indicate if the pain mechanism is limited to the peripheral nervous system, or if it is being maintained by the central nervous system (central sensitization). All the above may improve the treatment to be pursued. Although Quantitative Sensory Testing has been proved of high value in different orofacial pathologies, it has never been applied to AO. Many of the differences among people in the way they respond to therapies and their individual risk of developing side effects have been shown to be affected by their genetic makeup. We will search for these kinds of associations between inherited traits and development of neuropathic pain in this study. One important goal of this study is to understand what makes some people more likely to develop AO. Although genetic analysis will no directly help developing new treatments, we believe that the knowledge gained may ultimately help design better ways to predict and diagnose these conditions and to find more effective means of treatment. The proposed study findings can outline the genotype and sensory phenotype of AO patients and may suggest traits of patients undergoing RCT that are prone to develop AO. This exploratory study results may serve as the ground for a further study that will evaluate more patients prior to the procedure, will study predisposing factors and will help predict treatment outcome.